The present invention relates to certain polymerizable polyol(allyl carbonate) compositions and to polymerizates prepared from such compositions. Such polymerizates, e.g., articles such as optical lenses, are characterized by an improved resistance to yellowing when heated at elevated temperatures, e.g., at temperatures in the range of from about 80.degree. C. to about 180.degree. C.
Polymerizates prepared from aliphatic polyol(allyl carbonate) monomers, e.g., diethylene glycol bis(allyl carbonate) are characterized by hardness, impact resistance and optical clarity. For these reasons, optical articles, e.g., lenses, have been prepared from such polymerizates. Moreover, such lenses offer a weight advantage, vis a vis, glass lenses. It has also been proposed to prepare photochromic articles from such polymerizates. Frequently, photochromic articles are prepared by imbibing a photochromic substance within the preformed polymerizate, e.g., by immersion of the polymerizate in a hot solution containing the photochromic substance or by other thermal transfer mechanisms described in the art. It is also common to tint or dye such polymerizates (with or without the added photochromic substance) by immersion of the polymerizate in a heated aqueous dispersion of the selected dye.
When subjected to heat aging or heat treatments, such as those involved in tinting of the polymerizate or imbibition of a photochromic substance by thermal transfer, polymerizates prepared from polyol (allyl carbonate) compositions containing an aliphatic polyurethane, tend to take on a slight yellow cast. Such yellowing may adversely affect the commercial utility of such polymerizates, particularly in optical applications, and may, if significant, adversely affect the optical transparency and clarity of articles prepared from such polymerizates. There is, therefore, a continuing need for materials that are compatible with the polymerizable polyol(allyl carbonate) monomer composition and polymerizates prepared therefrom, which will prevent or mitigate yellowing of the polymerizate caused by heating at elevated temperatures, e.g. temperatures that may be used during post treatment of a pre-formed polymerizate. Such post treatment temperatures may range from about 80.degree. C. to about 180.degree. C., customarily from about 100.degree. C. to about 150.degree. C., e.g., 130.degree. C.
It has now been discovered that the addition of small amounts of a diakyl pyrocarbonate to a polymerizable polyol(allyl carbonate) monomer composition containing an aliphatic polyurethan component provides a polymerizate that exhibits enhanced resistance to yellowing caused by post formation heating.